In the fabrication of a self-aligned bipolar transistor, it is necessary to decrease a thickness of the base layer in order to realize a high frequency operation and to provide a sufficient break-down voltage. If a base layer is formed by ion implantation method, the problems such as channeling, quality-scattering due to low energy, implanation damage, etc. occur. Therefore, it is difficult to realize a self-aligned bipolar transistor having a thin base layer, if an ion implantation method is used to form the base layer.
Recently, a self-aligned bipolar transistor in which a base layer is formed by epitaxial technology has been proposed. According to epitaxial technology, it is possible to control a thickness and an impurity concentration of a base layer precisely.
A first conventional self-aligned bipolar transistor, which is described on pages 603 to 606 in a report "IEDM 90", includes a P.sup.- silicon substrate, an N.sup.+ epitaxial layer for a collector formed on the substrate, a P.sup.+ polycrystal silicon layer for a base electrode, which is formed on the N.sup.+ epitaxial layer and is provided with an aperture therein, a P-epitaxial base layer formed on the N.sup.+ epitaxial layer at the bottom of the aperture, a P-polycrystal silicon layer which is grown on an inner side surface of the aperture, an insulation layer formed on a surface of the P.sup.+ polycrystal silicon layer, a sidewall oxide layer formed on the inner side surface of the aperture which is covered with the P-polycrystal silicon layer, an N.sup.+ polycrystal silicon layer for an emitter electrode, which is formed in the aperture, and an N.sup.+ emitter layer formed on the P-epitaxial base layer by an impurity diffusion from the N.sup.+ polycrystal silicon layer. The aperture of the P.sup.+ polycrystal silicon layer is formed by dry-etching.
According to the first conventional self-aligned bipolar transistor, however, there are disadvantages in that the silicon substrate and the N.sup.+ epitaxial layer are damaged by the dry-etching process for forming the aperture. Further, a short circuit may result between the N.sup.+ polycrystal silicon layer and P.sup.+ polycrystal silicon layer, that is, between the base and emitter electrodes, because the sidewall oxide layer has a thin portion thereof at the corner of the P-polycrystal silicon layer.
A second conventional self-aligned bipolar transistor, by which the above disadvantges of the first conventional self-aligned bipolar transistor are overcome, is described on pages 607 to 610 in a report "IEDM 90".
The bipolar transistor includes a P.sup.- silicon substrate, an N.sup.+ epitaxial layer for a collector, an insulation oxide layer formed on the N.sup.+ epitaxial layer, a P.sup.+ polycrystal silicon layer for a base electrode, which is formed on the oxide layer and is provided with an aperture therein, a P-epitaxial base layer formed on the N.sup.+ epitaxial layer in the aperture by a selective growth, a P-polycrystal silicon layer which is grown on an inner side surface of the aperture at the same time the P-epitaxial base layer is grown for connecting the P-epitaxial base layer to the P.sup.+ polycrystal silicon layer, a sidewall insulation layer formed on the inner side surface of the aperture, an N.sup.+ polycrystal silicon layer for an emitter electrode, which is formed in the aperture covered with the sidewall layer, and an N.sup.+ emitter layer formed on a surface of the P-epitaxial base layer by an impurity diffusion from the N.sup.+ polycrystal silicon layer.
In the second conventional self-aligned bipolar transistor, the oxide layer is formed between the N.sup.+ epitaxial layer and the P.sup.+ polycrystal silicon layer. Therefore, when the aperture is formed in the P.sup.+ polycrystal silicon layer, the dry-etching process is carried out at the surface of the oxide layer. As a result, the silicon substrate and the N.sup.+ epitaxial layer are not damaged by the dry-etching process.
However, there is a disadvantage in that the contact area between the P.sup.+ polycrystal silicon layer and the P-epitaxial base layer is relatively small, so that a base contact resistance between them is increased if the P-polycrystal layer is not grown sufficiently.